To implement two-step velocity ratio power shift, a stepped automatic transmission generally employs two or more than two friction elements (clutch or brake), which are installed in a transmission in parallel, and controls the working states of the two or more than two friction elements (clutch or brake), thus a gear change without driving force interrupt can be achieved.
For the comparatively large volume of the friction element (clutch or brake), two or more than two friction elements (clutch or brake) will occupy a considerable space in the transmission, which is not beneficial for adding the gears by increasing the quantity of wheel gears; moreover, the friction element (clutch or brake) will produce a dragging resistance during idling, and the more the friction elements (clutch or brake) in the transmission are, the larger the dragging resistance produced is, that makes the gear efficiency of the transmission lower. Therefore, it is a development trend for the transmission to reduce the quantity of the friction elements (clutch or brake).